Chapter 7 - Energy Flow in an Ecosystem
1. Complete the following table (Carefully study the carbon, oxygen and nitrogen cycles).
| Bio-geo-chemical cycles | Biotic processes | Abiotic processes |
|---|---|---|
1. Carbon cycle | ||
2. Oxygen cycle | ||
3. Nitrogen cycle |
Ans:
| Bio-geo-chemical cycles | Biotic processes | Abiotic processes |
|---|---|---|
1. Carbon cycle | Photosynthesis, Respiration, Decomposition. | Burning of fossil fuels, forest fires, volcanic activity. |
2. Oxygen cycle | Photosynthesis, Respiration, Decomposition. | Combustion, Corrosion, rusting, formation of ozone (O₃). |
3. Nitrogen cycle | Biological nitrogen fixation, ammonification, nitrification, denitrification. | Atmospheric nitrogen fixation, industrial nitrogen fixation. |
2. Correct and rewrite the following statements and justify your corrections.
a. Carnivores occupy the second trophic level in the food chain.
Ans: False.
Correct Statement: Producers form the first trophic level in the food chain. Herbivores depend directly on producers. So they form the second trophic level whereas carnivores depend on herbivores so they form the third trophic level in the food chain.
b. The flow of nutrients in an ecosystem is considered to be a ‘one way’ transport.
Ans: False.
Correct Statement: The flow of nutrients in an ecosystem is cyclic. The nutrients are circulated and recycled from the biosphere to living organisms and after their death back to the biosphere.
c. Plants in an ecosystem are called primary consumers.
Ans: False.
Correct Statement: Plants are autotrophs. They produce their own food by the process of photosynthesis. All animals in the ecosystem directly or indirectly depend on plants for food.
3. Give reasons.
a. Energy flow through an ecosystem is ‘one way’.
Ans:
(i) The sun is the most important source of energy in any ecosystem.
(ii) The green plants of the ecosystem store some amount of solar energy in the form of food.
(iii) Before reaching the decomposers, this energy is passed on from one trophic level to the next.
(iv) Decomposers dissipate some amount of energy in the form of heat.
(v) However, no part of the energy ever returns to the sun. Hence, energy flow through an ecosystem is ‘one way’.
b. Equilibrium is necessary in the various bio-geo-chemical cycles.
Ans:
(i) The cyclic flow of nutrients within an ecosystem is called bio-geo-chemical cycles.
(ii) Nutrients necessary for the growth of organisms are continuously transferred from abiotic to biotic factors and from biotic to abiotic factors in an ecosystem.
(iii) Any imbalance in the cycles will break the link between the biotic and abiotic factors.
(iv) Therefore, equilibrium is necessary between bio-geo-chemical cycles.
c. Flow of nutrients through an ecosystem is cyclic.
Ans:
(i) All organisms need nutrients for their growth.
(ii) The nutrients carbon, oxygen, nitrogen, iron, calcium, etc. are circulated and recycled from the biosphere to living organisms and, after their deaths, back to the biosphere.
(iii) Nutrients are taken up by plants and then passed on to consumers.
(iv) Eventually, after their deaths, all types of consumers are decomposed by decomposers like bacteria and fungi, and the nutrients are again released into the biosphere and used again by living organisms.
(v) Therefore, the flow of nutrients through an ecosystem is cyclic.
| Dicots | Monocots | |
|---|---|---|
Seed | Two cotyledons | Single cotyledon |
Root | Well- developed primary root (Tap root) | Fibrous roots |
Stem | Strong, hard.
e.g., Banyan tree | Hollow.
e.g., Bamboo. False. e.g., Banana Disc-like e.g., Onion |
Leaf | Reticulate venation | Parallel venation |
Flower | Flowers with 4 or 5 parts or in their multiples (tetramerous or pentamerous) | Flowers that have three parts or are multiples of three (trimerous) |
4. Explain the following cycles in your own words with suitable diagrams.
a. Carbon cycle.
Ans:
(i) The circulation and recycling of carbon from the atmosphere to living organisms and, after their deaths, back to the atmosphere is called the carbon cycle.
(ii) Abiotic carbon atoms are circulated and recycled into biotic form mainly through photosynthesis and respiration. Hence, the carbon cycle is one of the most important bio-geochemical cycles.
(iii) Plants convert carbon dioxide into carbohydrates through the process of photosynthesis.
(iv) Similarly, they produce carbon compounds like proteins and fats, too.
(v) Carnivores feed on herbivores. In this way, biotic carbon is transported from plants to herbivores, from herbivores to carnivores, and from carnivores to apex consumers.
(vi) Main processes in the carbon cycle:
(vii) Eventually, after death, all types of consumer are decomposed by decomposers like bacteria and fungi, and carbon dioxide is released again into the atmosphere and used again by living organisms. In this way, carbon is continuously passed on from one living organism to another. After the death of living organisms, carbon goes into the atmosphere and is again taken up by living organisms.
b. Nitrogen cycle.
Ans:
(i) Nitrogen forms 78%, i.e., the maximum portion of the atmosphere. It is necessary for the maintenance of the cycle of nature.
(ii) The circulation and recycling of nitrogen gas into the form of different compounds through various biotic and abiotic processes in nature is called the nitrogen cycle.
(iii) All organisms participate in the nitrogen cycle. It is an important component of proteins and nucleic acids.
(iv) As compared to other elements, it is inactive and does not easily combine with other elements. Most organisms cannot use the free form of nitrogen.
(v) Important processes of the nitrogen cycle:
(a) Nitrogen fixation: Conversion of nitrogen into nitrates and nitrites through atmospheric, industrial, and biological processes
(b) Ammonification: Release of ammonia through decomposition of dead bodies and excretory wastes of organisms
(c) Nitrification: Conversion of ammonia into nitrite and then nitrate
(d) Denitrification: Conversion of nitrogen compounds into gaseous nitrogen
c. Oxygen cycle.
Ans:
(i) Oxygen forms 21% of the atmosphere. It is also present in the hydrosphere and lithosphere. The circulation and recycling of oxygen within the biosphere is called the oxygen.
(ii) This cycle includes both the biotic and abiotic components. Oxygen is continuously produced as well as used up in the atmosphere.
(iii) Oxygen is highly reactive, and it readily reacts with other elements and compounds.
(iv) As oxygen is found in various forms like molecular oxygen (O₂), water (H₂O), carbon dioxide (CO₂), inorganic compounds, etc., the oxygen cycle of the biosphere is extremely complex.
(v) Oxygen is released in the process of photosynthesis, whereas it is used up in processes like respiration, combustion, decomposition, corrosion, rusting, etc.
5. What would you do to help maintain the equilibrium in the various bio-geo-chemical cycles? Explain in brief.
Ans:
(i) Biogeochemical cycles always involve the achievement of equilibrium, i.e., a balance in the cycling of the nutrients between the spheres.
(ii) Human activities that are known to be environmentally unfriendly can disrupt this balance.
(iii) We should avoid deforestation, as trees play an important role in maintaining balance.
(iv) We should avoid overusing fertilizers. The fertilizers get washed away in the nearby water bodies, disrupting the balance.
(v) Avoid burning fossil fuels, as these fuels release a large amount of carbon in the form of carbon dioxide, thus disrupting the balance.
(vi) Vehicular emissions are another contributor to the disruption in the balance of the various cycles. These emissions release oxides of nitrogen and carbon and other hazardous air pollutants. So we must use better-quality fuel like CNG or ethanol to reduce vehicular emissions.
6. Explain in detail the inter-relationship between the food chain and food web.
Ans:
(i) Interactions go on continuously between producers, consumers, and decomposers.
(ii) There is a definite sequence to these interactions, which is called the food chain.
(iii) Each chain consists of four, five, or more links.
(iv) An ecosystem consists of many food chains that are interconnected at various levels. Thus, a food web is formed.
(v) An organism may be the prey of many other organisms.
(vi) For example, an insect feeds upon the leaves of various plants, but the same insect is the prey of different animals like frogs, wall lizards, birds, etc.
(vii) Thus, many food chains interconnected together form an intricate web called a food web.
7. State the different types of bio-geo-chemical cycles and explain the importance of those cycles.
Ans:
(i) The different types of bio-geo-chemical cycles are nitrogen, oxygen, carbon, water vapor, iron, calcium, phosphorus, etc.
(ii) Nutrients necessary for the growth of the organisms are continuously transferred from abiotic to biotic factors and from biotic to abiotic factors within an ecosystem.
(iii) These cycles operate continuously through the medium of the biosphere, formed by the lithosphere, atmosphere, and hydrosphere.
(iv) The recycling of biological, geological, and chemical sources of nutrients takes place through these cycles.
(v) Nutrients from the biosphere enter the bodies of plants and animals. Eventually, after death, all types of consumers are decomposed by decomposers like bacteria and fungi, and they are again released into the biosphere and used again by living organisms.
(vi) Therefore, these cycles help in maintaining the flow of nutrients and energy through the ecosystem and maintaining equilibrium in the ecosystem.
8. Explain the following with suitable examples.
a. What type of changes occur in the amount of energy during its transfer from plants to apex consumers?
Ans:
(i) Plants in the ecosystem store some of the solar energy in the form of food.
(ii) Before reaching the decomposers, this energy is passed on from one trophic level to the next.
(iii) At every trophic level, some amount of energy is used by the organism for its own life processes and some amount of energy is lost to the surroundings.
(iv) Decomposers dissipate some amount of energy in the form of heat.
(v) However, no part of the energy ever returns to the sun. Hence, such a passage of energy is referred to as ‘one-way’ transport.
(vi) Therefore, energy is maximum at the base of the pyramid and is least at the apex; e.g., phytoplankton, which form the base of the pyramid, have 10,000 kcal of energy, while humans at the apex have 10 kcal of energy.
b. What are the differences between flow of matter and of energy in an ecosystem? Why?
Ans:
| Flow of matter | Flow of energy |
|---|---|
(i) It involves the circulation and recycling of nutrients in a cyclic manner within the biosphere. | (i) It involves the flow of energy from one trophic level to another in a unidirectional or noncyclic manner. |
(ii) There is no dissipation of matter at any level. | (ii) There is dissipation of energy at every level. |
(iii) The biosphere is the source of nutrients. | (iii) The Sun is the most important source of energy. |